This invention is directed to a method of reducing the level of non-volatile mutagens in chlorinated water by sulfite treatment. Water to be used for drinking is commonly purified by chlorination to destroy any harmful microorganisms which may be present. It has been shown that non-volatile direct acting mutagens (not requiring enzymatic activation) are generated by the water treatment process of chlorination. There is concern that some of the mutagens produced may be carcinogenic. Although alternative water treatment methods are known, such as ozonation, which may or may not produce mutagens, the vast majority of water treating facilities are adapted to use chlorination. It has been discovered that treatment of chlorinated water with sulfite reduces mutagen levels significantly.